The pursuit of petroleum products in deep waters has revealed an underwater world completely different from a level or gradually sloping seabed. Far off the coast, unlike relatively featureless continental shelves where most offshore oil and gas has been historically developed, the deep-water ocean bottom has hazardous topographic features that can compromise pipelines and subsea structures. These topographic features include enormous basins, domes, valleys, cliffs, canyons, and escarpments.
An escarpment, or scarp, is a steep slope or cliff formed by erosion or faulting. The Sigsbee Escarpment, for example, is the largest in the Gulf of Mexico and lies beyond the edge of the continental shelf thousands of feet below the sea surface. The Sigsbee Escarpment encompasses drops of hundreds to over a thousand feet and extends for hundreds of miles. Between the Sigsbee Escarpment and the continental shelf exists a region called the continental slope. Because of the randomness and variability of the salt and sediment deposits, the topography of the continental slope is a complex landscape with many scarp-like features.
This complex topography is a significant challenge to laying subsea pipeline across these regions. The abrupt changes in the slope across such topographic features and escarpments can cause pipelines crossing them to bend sharply. This bending leads to ovalization of the pipeline cross section which may cause the pipeline to buckle and collapse. Large free spans exceeding the stress and vortex induced vibration fatigue limits of the pipeline can also result from seabed irregularities associated with these topographic features.
Subsea pipelines are most often used to transport production fluids from offshore facilities to land or to other offshore facilities. Such fluids include, but are not limited to, gases (methane, ethane, etc.), liquid hydrocarbons, additives (diluents added to heavy fluids, or corrosion control additives), or any mixture thereof. These pipelines typically rest on the ocean bottom and can extend for miles at depths exceeding 3500 feet (1050 m) of water. Many issues arise with respect to the laying of subsea pipelines including countering the subsea currents, traversing the varying topography, and the complexity of the installation process itself. Existing solutions for spanning the treacherous topographic features described above can be costly, risky, environmentally destructive, or result in other hazards.
Existing solutions include re-routing pipelines through existing valleys or canyons where the slope is more gradual, drilling subsea conduits, and blasting or trenching the undersea topography to provide a better support profile for the pipeline. The re-routing option can be time consuming and expensive because it requires a longer pipeline. The trenching, blasting, and drilling options can have a negative impact on the undersea environment and sea life and can likewise be very costly. Other options, including the installation of rigid pilings and framework to support pipeline spans have been tried on smaller scale installations, but would be very costly on longer spans.
Undersea pipelines are crucial to the low cost delivery of production fluids (hydrocarbons) from offshore facilities to land or to other offshore facilities. If pipelines are not available, the hydrocarbons must be transported via tankers or some other means to the coast. Pipelines are generally considered lower risk than tankers because there is significantly less risk of maritime collisions and there are fewer exchanges (platform to tanker; tanker to shore facility) of the hydrocarbons. The hazardous topography of the continental slopes increases the risk (through stresses and failures) that leaks may occur. A solution that safely allows pipelines to traverse hazardous topography in a manner that is more cost effective and environmentally responsible would be highly desirable.